1. Field of the Invention
The present invention relates to a photoelectric distance sensor for sensing distances to objects by irradiating light to the objects and by receiving the reflected light from the objects.
2. Description of the Prior Art
FIG. 1 is a block diagram drawn with single lines for explaining the basic operation of a conventional photoelectric distance sensor for sensing positions. In FIG. 1, reference numeral 1 denotes a photoelectric distance sensor, and numeral 2 denotes an optical system mainly composed of lenses. Numeral 3 denotes a light-irradiating section mainly composed of a light-emitting device such as an light-emitting diode- Numeral 4 denotes a light-receiving section arranged on a light-receiving surface and mainly composed of light-receiving devices. Numeral 5, 6 and 7 denote a signal-processing section, a sense object and a background respectively.
FIG. 2 is a longitudinal section of a light-position-sensing device 10 arranged in a direction connecting a light-receiving axis with a light-irradiating axis on the light-receiving surface. As the light-position-sensing device 10, the disclosed in Patent Publication Gazette No. 42411 of 1983 is adopted, for example. It has the characteristic shown in FIG. 3.
The operation of the photoelectric distance sensor 1 will now be described. Light from the light-irradiating section 3 is irradiated on the sense object 6 through the light-irradiating axis, and forms an bright spot on the sense object 6. Light reflected by the sense object 6 travels into the light-receiving section 4 through the light-receiving axis. In the light-receiving section 4, bright spot images are formed on the light-receiving surface at positions corresponding to distances to the sense object 6. When light travels into a certain position on the light-position-sensing device 10 located on the light-receiving surface in order to determine the bright spot positions on the light-receiving surface with an aim of determining distances X to the sense object 6, two current outputs Ia and Ib are obtained from the light-position-sensing device 10, as shown in FIG. 2. Light incidence positions Y into the light-position-sensing device 10 are obtained by the next relational expression, where an effective length of the light-position-sensing device 10 is denoted by L. EQU (Ia-Ib) / (Ia+Ib)=2Y/L (1)
The relational expression is depicted in FIG. 3.
The relation between the distances X and the light incidence positions Y is provided in a next relational expression. EQU XY=L2L3 (2)
In the expression, L2 denotes light axis pitches between incident lights and received lights, and L3 denotes distances between the light-receiving device and the lens. Because the light-position-sensing device 10 can determine the light incidence positions Y electrically as described above, and then can deduce distances X, it is widely used.
On the other hand, one of factors varying accuracies of the distances is differences of reflectances of the objects. That is, there was a problem that distance measurement errors between high reflectance objects such as white objects and low reflectance objects such as black objects being at the same distance were different in each other.
A method resolving the problem is to control irradiated light intensity to have receiving light intensity constant (for example, as disclosed in Laying-open Publication No. 39470 of 1974 and Patent Publication Gazette No. 42411 of 1983).
Because the method controls the irradiated light intensity so that the incident light amounts to the light-receiving section may be always constant, the incident light into the light-receiving section is not influenced by reflectances of objects and becomes constant. Therefore it has a superior effect that it can reduce errors occurred from reflectance differences at the same distance.
The resolving method has the superior feature above mentioned, however it has a problem which will be described as follows, on the other hand.
FIG. 5 is a characteristic diagram showing received light amount variations when irradiated light amounts are constant. FIG. 6 is a characteristic diagram showing the relation between sense distances and target values of the received light amounts. FIG. 7 is a characteristic diagram showing the relation between the sense distances and the irradiated light amounts.
The target values of the received light amounts cannot become larger than the received light amount from the object having the smallest reflectance and located at the largest distance within the distance-measurable range. Wide distance-measurable ranges make the target values of the received light amounts small.
That is, the received light amounts in the case where the irradiated light amounts are constant vary much, as shown in FIG. 5. in FIG. 5, region A is the distance-measurable range, region B is a nearest range, and region C is a distant unmeasurable range. Suppose that for example, received light amounts can be regarded to be inversely proportional to squares of distances in the region A, the distance-measurable range. Accordingly, the target values of the received light amounts cannot become larger than the values obtained from the smallest reflectance objects being at distant positions.
The wider the distance-measurable ranges become, the more serious the problem becomes. For example, if the distance of the sense object becomes three times wider, the variations of the received light amounts become one ninth under the condition of constant irradiated light amounts. In addition to this, reflectance differences of sense objects' surfaces exist. For example, white objects' reflectances differ from those of black objects in about ten times. Then, the received light amount from a black object on the most distant position becomes one ninetieth compared to that from a white object on the nearest position. Accordingly, because the target value of received light amount cannot be become larger than the value, the irradiated light amount becomes small to one ninetieth in the case where a white object exists on the nearest position. FIG. 6 and FIG. 7 depict this fact.
That is, the measurement accuracy of photoelectric distance sensors having wide distance-measurable ranges becomes worse than that of photoelectric distance sensors having narrow distance-measurable ranges, even in case of measuring the same object on the same distance. Displacement sensors outputting the displacement from predetermined distances essentially contain the entirely same problem as mentioned above.